1. Field of the Invention
The present invention relates to a zoom lens system, and particularly relates to a compact zoom lens system capable of taking both telephoto and wide angle shots, of which the overall length is fixed and shortened for use in a camera phone, a compact digital camera, etc.
2. Description of Prior Art
It is well known that, due to the limited space, photographing lenses used in camera phones and compact digital cameras have dimensions much smaller than those in photographic cameras, video cameras and common digital cameras. Accordingly, camera phones and compact digital cameras generally employ fixed focal length lenses, which are disclosed in, for example, U.S. Pat. No. 6,650,486.
Recently, with the rapid development of the solid state image pickup device used in cameras, such as a CCD (Charge-Coupled Device) sensor or a CMOS (Complimentary Metal-Oxide Semiconductor) sensor, image resolutions for camera phones have been increased from 0.1 million pixels, 0.35 million pixels to 1.3 million pixels and even 2 million pixels. However, since the focal length of a fixed lens is fixed rather than adjustable, the angle of view that the picture takes in is also set. Thus, the photo taker has to move closer or further to the object to be photographed or change lenses. In the former case, it may not be possible to get noticeably closer. Accordingly, it is very inconvenient for the photo taker to use a fixed lens for photography. In addition, when digital zoom is applied, the resulting image will be degraded. As is well known, the main disadvantage of digital zoom is that the more that the photo taker zooms in with the digital zoom, the greater the enlargement and the lower the image quality. Therefore, currently, optical zoom using a number of lenses is still more preferred.
Various designs of optical zoom lenses have been proposed. U.S. Pat. Nos. 6,191,896 and 6,233,099 assigned to Canon disclosed a four-group zoom lens system having, from the object side thereof, a negative-positive-negative-positive refractive power configuration. During zooming, all the lens groups move along the optical axis in such a manner that the distance between every two-lens groups is changed. US Patent Application Serial No. 20030012567 discloses a three-group zoom lens system having, from the object side thereof, a negative-positive-positive refractive power configuration. Zooming is effected by varying the distance between the first and second lens groups, and the third lens group is not moved during zooming. However, during focusing and zooming, the overall length of each of the conventional zoom lenses as described above is changed. When the position of the first lens group varies upon zooming, working distance and the focusing position varies in the close-up focusing range, so it is not so easy to handle and use. Further, each of the above conventional zoom lens system is composed of at least six single lenses, and thus is difficult to incorporate into a mobile phone with a thickness generally less than 20 mm. Also, the production cost and weight are increased, thereby contradicting the compactness trend. Even the above conventional zoom lens system is incorporated into the mobile phone, accident impact on or damage to the zoom lens system may be caused during operation since the overall length varies during focusing and zooming. In addition, since the front lens barrel is rotated during focusing, it is inconvenient for the photo taker to use a circular polarizing filter and Petal-type hood.
It is reported that, Philips Research has demonstrated a unique liquid zoom lens system that alters its focal length by changing its shape. The Philips liquid lens consists of two immiscible (non-mixing) fluids of different refractive index, one an electrically conducting aqueous solution and the other an electrically non-conducting oil, contained in a short tube with transparent end caps. Without an electrical charge, the surface of the conducting liquid forms a curve. But when a charge is applied through the electrodes, the surface tension of the liquid changes, altering its curvature and thus the focal point of light passing through it. Different voltages produce different curvature changes. However, the operational durability, the optical performance and the lifespan of this unique zoom lens system remain to be further tested. Therefore, its rapid introduction into high-volume manufacturing cannot be expected.